In recent years, technologies relating to thin-film solar cells have been advanced to realize inexpensive and light-weight solar cells and, therefore, thinner solar cells manufactured with less material have been demanded. In addition, such thin-film solar cells are demanded for scientific reasons, and the influence on the characteristics of a solar cell due to a reduction in thickness are of interest.
Disclosed in "Technical Digest of the International PVSEC-5, Kyoto, Japan, 1990" pp. 943-946 is a thin-film solar cell in which portions of an active layer are selectively etched away from the rear surface leaving portions serving as a honeycomb reinforcing structure.
FIGS. 26(a) and 26(b) are plan views for explaining a prior art thin-film solar cell, in which FIG. 26(a) illustrates a solar cell module and FIG. 26(b) illustrates a thin-film solar cell included in the solar cell module. In the figures, the solar cell module 200a includes a plurality of thin-film solar cells 200 arranged in a matrix on a substrate 200b. Each of the thin-film solar cells has comb-shaped surface electrodes 10a and 10b on the front surface and a rear electrode 20 on the rear surface. In the solar cell module 200a, a plurality of solar cells 200 are connected in series so that the surface electrodes 10a and 10b of each solar cell are connected to the rear electrode of the adjacent solar cell using a wire 11.
The comb-shaped surface electrode 10a (10b) comprises a common bus electrode 10a.sub.1 (10b.sub.1) and a plurality of grid electrodes 10a.sub.2 (10b.sub.2) protruding from opposite sides of the bus electrode. The bus electrodes 10a.sub.1 and 10b.sub.1 are parallel to each other. In operation, photocurrent generated on the surface of the thin-film solar cell 200 is collected by the grid electrodes 10a.sub.2 and 10b.sub.2 and transferred to the respective bus electrodes 10a.sub.1 and 10b.sub.1.
FIG. 29(e) illustrates a cross-sectional view of the thin-film solar cell 200 taken along line 29e--29e of FIG. 26(b). In FIG. 29(e), reference numeral 201 designates a p type monocrystalline silicon (Si) substrate about 150 .mu.m thick. An n type region 202 is disposed within the surface region of the p type monocrystalline Si substrate 201, producing a p-n junction. That is, the substrate 201 serves as an active region performing light-to-electricity conversion in the vicinity of the p-n junction. The comb-shaped surface electrodes 10a and 10b are disposed on prescribed portions of the n type region 202 spaced apart from each other. An anti-reflection film (hereinafter referred to as AR film) 225 comprising a lower SiN film 204a and an upper SiO.sub.2 film 223 is disposed on the surface of the n type region 202 where the surface electrodes 10a and 10b are absent. The AR film 225 confines incident light in the active layer.
The Si substrate (active region) 201 has a honeycomb reinforcing structure 210 on the rear surface, and the reinforcing structure 210 improves the mechanical strength of the thin Si substrate 201. The height of the reinforcing structure 210 is about 150 .mu.m.
A rear electrode 206 is disposed on a part of the rear surface of the Si substrate 201 parallel to the surface electrodes 10a and 10b. A p.sup.+ type BSF (Back Surface Field) region 203 is disposed within the Si substrate 201 contacting the rear electrode 206. The p.sup.+ type BSF region 203 produces an energy barrier against minority photocarriers (electrons) in the vicinity of the rear electrode 206. The energy barrier prevents the photocarriers from reaching the interface between the substrate 201 and the rear electrode 206, whereby disappearance of photocarriers at the interface is avoided and the photocarriers generated at the interface are accelerated toward the surface electrodes.
A method for manufacturing the thin-film solar cell is illustrated in FIGS. 27(a)-27(d), 28(a)-28(c), and 29(a)-29(e).
Initially, an acid-proof photoresist 221 is patterned on the opposite front and rear surfaces of the Si substrate 201, on which an oxide film (not shown) is produced, except for a prescribed region on the rear surface of the substrate, preferably using a screen printer (FIG. 27(a)). In the figure, the acid-proof photoresist pattern on the front surface of the substrate is omitted. After drying the acid-proof photoresist pattern 221, the Si substrate 201 is etched from the rear surface using the acid-proof photoresist pattern 221 as a mask, whereby the Si substrate 201 is thinned leaving the honeycomb reinforcing structure 210 (FIG. 27(b)). A mixture of hydrofluoric acid and nitric acid is employed as an etchant. The patterning of the acid-proof photoresist may be carried out by a conventional photolithographic process.
After removing the photoresist pattern 221, phosphorus is diffused into the Si substrate 201 from the front and rear surfaces using POCl.sub.3 (phosphorus oxychloride), forming n type regions 202a and 202b. A p-n junction is produced at the interface between the p type Si substrate 201 and each of the n type regions 202a and 202b (FIG. 27(c)).
The front surface n type region 202a is covered with an acid-proof photoresist 222, and the rear surface n type region 202b is etched away using hydrofluoric acid. Thereafter, A1 paste 230 is selectively screen-printed on a center part of the rear surface of the substrate 201, and an annealing is carried out at 600.degree..about.800.degree. C. to diffuse A1 into the Si substrate, forming a p type high impurity concentration region, i.e., the p.sup.+ type BSF region 203 (FIG. 27(d)).
After removing the acid-proof photoresist 222 from the front surface of the Si substrate 201, SiO.sub.2 and TiO.sub.2 are successively deposited on the surface by LPCVD (Low Pressure Chemical Vapor Deposition), forming an AR layer 204 having a two-film structure (FIG. 28(a)). Then, surface electrodes 10a and 10b are formed by firing an Ag paste. More specifically, Ag paste 10 is selectively screen-printed on prescribed portions of the AR film 204 (FIG. 28(b)). When the Ag paste 10 is baked, the Ag paste penetrates through the AR layer 204 comprising the SiO.sub.2 film and the TiO.sub.2 film and reaches the n type region 202, resulting in the surface electrodes 10a and 10b electrically in contact with the n type region 202 (FIG. 28(c)).
Finally, Ag paste is screen-printed on a part of the rear surface of the substrate opposite the BSF region 203 and it is baked to form the rear electrode 206 (FIG. 28(c)).
While in the above-described process steps of FIGS. 28(a)-28(c) the AR layer 204 comprises the SiO.sub.2 film and the TiO.sub.2 film, the AR layer may comprise an SiN film. FIGS. 29(a)-29(e) illustrate process steps employing the SiN AR layer.
After forming the BSF region 203 as shown in FIG. 27(d), SiN is deposited on the n type region 202 by plasma CVD, forming an AR layer 204a (FIG. 29(a)).
Thereafter, an SiO.sub.2 film 223 is deposited on the SiN AR layer 204a by LPCVD, and an acid-proof photoresist 224 is screen-printed on the opposite front and rear surfaces of the Si substrate 201 except a prescribed region of the front surface (FIG. 29(b)). In the figure, the acid-proof photoresist on the rear surface is omitted. After drying the acid-proof photoresist 224, the SiO.sub.2 film 223 is selectively etched with hydrofluoric acid using the acid-proof photoresist 224 as a mask (FIG. 29(c)).
After removing the acid-proof photoresist 224, the SiN AR layer 204a is selectively etched with phosphoric acid using the SiO.sub.2 film 223 as a mask (FIG. 29(d)).
In the step of FIG. 29(e), Ag paste is screen-printed on portions of the front surface where the n type region 202 is exposed and it is baked to form the surface electrodes 10a and 10b. Finally, Ag paste is screen-printed on a part of the rear surface of the substrate opposite the BSF region 203 and it is baked to form the rear electrode 206.
In the above-described method for producing the thin-film solar cell, however, since the monocrystalline Si substrate 201, which is a high purity material, is selectively etched from the rear surface leaving the honeycomb reinforcing structure 210, a thick monocrystalline Si substrate including the thickness of the reinforcing structure 210 is required. Although this substrate thinning process provides a light-weight solar cell, the required quantity of the monocrystalline Si cannot be reduced. Therefore, it is difficult to reduce the production cost by reducing the material cost.
While in the above-described prior art solar cell the thin Si substrate is supported by the honeycomb reinforcing structure formed on the rear surface of the substrate, Japanese Published Patent Application No. 4-91482 discloses another structure for supporting the thin Si substrate. In the prior art, after forming a thin Si film, an AR film, and a comb-shaped surface electrode successively on a front surface of a low purity Si substrate, a cover glass is adhered to the front surface of the structure using a transparent adhesive, such as EVA (ethylene vinyl alcohol). Then, the low purity Si substrate is etched from the rear surface using a KOH (potassium hydroxide) solution until an oxide film serving as an etch stop is exposed from the rear surface. After the oxide film is etched away using hydrofluoric acid, A1 is deposited on the rear surface by sputtering and baked to form a rear electrode and a BSF region opposite the rear electrode. The low purity Si substrate supports the thin Si film during the process steps on the front surface.
When a solar cell module is fabricated using a plurality of thin-film solar cells produced as described above, the surface electrode of each solar cell covered with the glass plate must outside the solar cell before the wire-bonding process connecting the solar cells. Since the solar cells are arranged in a matrix the modularization efficiency is reduced.
A description is now given of a method for forming an AR film employed in the above-described prior art thin-film solar cell.
FIGS. 30(a)-30(c) are sectional views illustrating process steps of forming an aperture pattern in an AR film.
Initially, an AR film 305 about 800 .ANG. thick comprising SiN is formed on a Si substrate 301 in which a p-n junction active region 301a is formed, and an acid-proof photoresist 306 having an aperture 306a is screen-printed on the AR film 305 (FIG. 30(a)).
Using the acid-proof photoresist 306 as a mask, the AR film 305 is selectively etched with heated phosphoric acid, forming an aperture 305a (FIG. 30(b)). After removing the acid-proof photoresist 306, a surface electrode 310 is formed on the p-n junction active region 301a exposed in the aperture 305a (FIG. 30(c)).
In the above-described process steps of forming the AR film, however, since the aperture 305a of the AR film 305 is formed by the selective etching using the acid-proof photoresist 306 as a mask, the acid-proof photoresist 306 must be removed using an organic solvent or the like after the formation of the aperture 305a, complicating the production process. Since it is difficult to completely remove the acid-proof photoresist 306, the surface of the AR film 305 after the removal of the photoresist 306 is contaminated with the photoresist material, decreasing the quantity of light incident on the p-n junction active region 301a.
Where the AR film is of a two-layer structure to improve the effect of confining incident light in the p-n junction active region 301a, in addition to the above-described problems, the steps of forming the upper AR film and an aperture in the upper AR film are required, further complicating the production process.
A description is now given of a conventional etching process employed in the above-described method for producing a thin-film solar cell.
FIG. 31 is a schematic diagram illustrating a conventional automatic etching apparatus. In the figure, the automatic etching apparatus 400 includes a container 404 in which samples 407, such as semiconductor wafers, are put (hereinafter referred to as sample container), a conveyer 401 for conveying the sample container 404, and a controller 403 for controlling the conveyer 401. Reference numeral 405 designates a flask filled with an etchant 406.
The sample 407 is a device including a metal layer, semiconductor layer, insulating film, and the like, such as the above-described thin-film solar cell. The sample container 404 and the flask 405 comprise quartz or stainless steel (SUS). The controller 403 includes a timer 408 for setting the etching time.
The sample container 404 is put in the flask 405 filled with the etchant 406 by the conveyer 401. After the etching time set by the timer 408, the sample container 404 is taken out of the flask 405 by the conveyer 401. Since the etching time is determined according to a previously measured etching rate, the etching depth in the sample 407 can be controlled by the etching time.
The operation of the etching apparatus 400 will be described in more detail.
The conveyer 401 conveys the sample container 404 containing the sample 407 to the flask 405 and immerses the container 404 in the etchant 406 according to the control signal from the controller 403. Then, the etching of the sample 407 is started. After the etching time set by the timer 408, the conveyer 401 pulls out the sample container 404 from the flask 405 according to the control signal from the controller 403. Then, the etching of the sample 407 is finished.
In the conventional automatic etching apparatus, even if the etching time set by the timer 408, i.e., the time during which the sample 407 is immersed in the etchant 406, is fixed, the etching depth in the sample unfavorably varies because the etching rate varies depending on the etchant and the sample.